Evol Ecol Res 7: 1089-1107 (2005)     Full PDF if your library subscribes.

Plasmids survive despite their cost and male-specific phages due to heterogeneity of bacterial populations

Francisco Dionisio*

Instituto Gulbenkian de Ciência, Apartado 14, P-2781-901 Oeiras, Portugal


e-mail: dionisio@igc.gulbenkian.pt

ABSTRACT

Problem: Conjugative plasmids manipulate their bacterial hosts to express sex-pili – long, filamentous organelles whose genes are carried by the plasmids themselves. Sex-pili locate and attach to another bacterial cell for plasmid transfer. Bacteria differ in their ability to express sex-pili and hence to donate plasmids. Does this diversity in sex-pilus expression contribute to the survival of conjugative plasmids?

Method: Simulations of a chemostat containing bacterial cells, some harbouring a conjugative plasmid. Certain viruses, called male-specific phages (MSPs), enter the chemostat. These viruses infect only donor cells. I monitor whether plasmids continue to survive in each simulation.

Key assumption: A bacterium’s ability to donate is proportional to its susceptibility to MSPs.

Conclusions: Bacterial diversity in sex-pilus expression strongly increases the size of the parameter space for plasmid survival. A host with low expression of sex-pili – and hence a low plasmid transfer rate – represents a sink habitat for the conjugative plasmid. However, hosts with high expression of sex-pili also constitute sink habitats – although they have a high plasmid transfer rate, they are the most susceptible to MSPs. A system consisting of both types of bacteria (two sink habitats) constitutes a source habitat for the conjugative plasmid. At the same time, this diversity helps to maintain the MSP itself because extinction of the conjugative plasmids would also doom the MSP.

Keywords: maintenance of plasmids, male-specific-phages, polymorphism, sink habitat, source habitat.

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        © 2005 Francisco Dionisio. All EER articles are copyrighted by their authors. All authors endorse, permit and license Evolutionary Ecology Ltd. to grant its subscribing institutions/libraries the copying privileges specified below without additional consideration or payment to them or to Evolutionary Ecology, Ltd. These endorsements, in writing, are on file in the office of Evolutionary Ecology, Ltd. Consult authors for permission to use any portion of their work in derivative works, compilations or to distribute their work in any commercial manner.

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